The present invention relates to a process for producing poly-xcex1-olefin compositions, in particularly polypropylene copolymer compositions having uniform quality with desired stiffness and impact strength properties and being suitable for a wide range of applications including different packaging applications.
It is known that especially polypropylene polymers have suitable resistance to heat and chemicals, and they also have attractive mechanical properties. Further, it is known that desired properties, e.g. stiffness and impact strength properties of polypropylene can be achieved by copolymerizing propylene with ethylene or other alfa-olefin monomers and optionally by adding elastomeric components to the copolymer matrix. Polypropylene copolymers can thus be used as alternatives e.g. for soft poly(vinyl chloride) (PVC). Further, polypropylene homo- or copolymers are very suitable in a wide range of applications, where emissions of chlorinated organic compounds and many other emissions should be strictly restricted.
By xe2x80x9cxcex1-olefin monomerxe2x80x9d in this connection is meant an xcex1-olefin which is capable of polymerization by the insertion (Ziegler-Natta) mechanism. An xcex1-olefin is a compound having the structure CH2xe2x95x90CHR, wherein R is a linear, branched or cyclic alkyl group. In particular R is a linear or branched alkyl group having 1 to 12 carbon atoms or a cyclic alkyl group having 4 to 8 carbon atoms. Typical xcex1-olefin monomers used in the present invention are propylene, 1-butene, 4-methylpentene, 1-hexene and octene. Preferably xcex1-olefin is propylene. Said xcex1-olefins are copolymerized with ethylene and optionally with other xcex1-olefins, such as butene.
Typically xcex1-olefin copolymers are nowadays prepared with a multiphase process comprising one or more bulk and/or gas phase reactors in the presence of Ziegler-Natta catalyst system comprising a catalyst comprising a compound of a transition metal belonging to groups 4 to 6 of the Periodic Table of Elements (IUPAC 1990), and a cocatalyst based on an organic compound of a metal belonging to any of groups 1 to 3 and 13 of said Table. Typical compounds of transition metals are the chlorides, especially the tetrachloride of titanium. Typical organometallic cocatalysts are organoaluminium compounds such as aluminium alkyl compounds and especially trialkyl aluminiums. Further, this kind of catalyst system has been developed by depositing and thus solidifying the transition metal compound on a more or less inert and particulate support and by adding to the catalyst composition in the stages of its preparation several additives, among others internal and external electron donors, which act as stereoregulating agents. A typical support is magnesium chloride, typical internal electron donors are the dialkyl phthalates and typical external electron donors are the alkyl alkoxy silanes. These compounds have improved the polymerization activity, the operating life and other properties of the catalyst system and above all the properties of the polymers which are obtained by means of said catalyst system. In order further to improve the properties of such a catalyst system at least a part of it has been contacted with a small amount of monomer to give a polymer coated, so called prepolymerized catalyst or catalyst system.
The multiphase polymerization process can comprise several polymerization stages. It is common knowledge that the polymer matrix, which comprises homo/homo, homo/random or random/random (co)polymers with a comonomer content needed to obtain the desired properties, can be prepared in the first stage of the polymerization process. The first stage can comprise bulk phase and optionally gas phase reactor(s). Very often the first stage comprises one bulk and one gas phase reactor. Advanced heterophasic copolymers can be obtained, if one or more additional gas phase reactors, which are often called as rubber phase reactors, are used, combined in series with the first stage reactor(s). Copolymerization of ethylene monomer and xcex1-olefins in the presence of the polymer matrix from the first stage is carried out in the rubber phase reactor(s), which step forms the second stage of the polymerization process.
Fouling is a common problem during the olefin polymerization process. Fouling occurs when product from the bulk reactor is transferred forward, e.g. to the flash and to the gas phase reactor for further polymerizing. The polymer product which is to be transferred is xe2x80x9ctackyxe2x80x9d or xe2x80x9cstickyxe2x80x9d and adheres to the walls of the reactor and other surfaces in flash and gas phase reactors. Further, in the gas phase reactor fouling occurs due to the static electricity caused by tacking of the charged polymer particles on the walls. Detrimental fouling is caused in the gas phase reactors by the fines. i.e. very small particles containing active catalyst. Such particles are often called xe2x80x9chotxe2x80x9d catalyst particles. Especially the fouling caused by the rubbery fines in the rubber phase reactor is very detrimental for the process and the products.
It is known that the degree of fouling can to some extent be restricted by adding various antistatic chemicals to the first stage reactors. Further, addition of catalyst deactivation chemicals, known as catalyst xe2x80x9ckillersxe2x80x9d, results in killing or reducing of the catalyst activity which, in turn, reduces the formation of the tacky material.
EP Patent 669 946 discloses a two stage gas phase process for producing polypropylene copolymer wherein a gel reduction component is introduced into the first stage reactor for preventing fouling in the second stage reactor. Said gel reduction component is an electron donor and acts as a catalyst deactivator. As most preferred deactivators are cited alkylene glycols and derivatives thereof, but also methanol and ethanol are mentioned.
In U.S. Pat. No. 4,182,810 there is disclosed a method of reducing fouling during particle form polymerization of ethylene, carried out in hydrocarbon diluent, and typically in loop reactors. The fouling is caused by the adherence of polymer particles to the walls of the polymerizing reactors and reduced by adding to the reaction medium a composition comprising a mixture of polysulfone copolymer, a polymeric polyamine, an oil-soluble sulfonic acid and toluene. A typical antifouling agent containing this kind of mixture is supplied by DuPont under the trade name Stadis 450.
U.S. Pat. No. 5,026,795 describes a process for preventing fouling in a one stage gas phase copolymerization reactor by admixing an antistatic agent with a liquid carrier comprising comonomers such as 1-hexene, and introducing said mixture to the reactor. Ethylene is used as a monomer. The antistatic agent used comprises Stadis 450 type agents. It should be pointed out that only a very small amount of antistatic agent is necessary, otherwise the antistatic agent would poison or deactivate the catalyst which, according to the cited patent, is not desired.
Further, it is known that tertiary amine compositions and compositions of olefin-acrylonitrile copolymer and polymeric polyamines have been used as antistatic agents for preventing fouling in polymerization processes, especially in ethylene polymerization processes.
As the above discussion shows, a wide range of modification possibilities of polypropylene results in a wide range of applications where polypropylene can be used. However, many applications, such as medical and food packaging applications set limits to the allowable residues and emissions of the used raw materials, additives and adjuvants. Therefore, the use of many known antistatic agents is limited for a wide range of applications.
xe2x80x9cStadis-typexe2x80x9d agents, such as Stadis 450, have been used as antistatic agents in polyethylene processes as is shown in the examples of the above U.S. Patents. The antistatic effect of xe2x80x9cStadis-typexe2x80x9d agents may, as such, be sufficient to to reduce fouling in polyethylene polymerizing processes. However, when xcex1-olefins are polymerized, more complicated catalyst systems are used, which in turn complicates the fouling phenomena. Polymerizing of xcex1-olefins in this connection comprises also the copolymerizing processes of xcex1-olefins with ethylene and/or optionally with other xcex1-olefins. As xcex1-olefins are preferably used propylene monomers. As was discussed above, the fouling problem in propylene multistage polymerizing processes comprising bulk and optionally gas phase reactor(s) and as well as rubber phase reactor(s) is a result not only of static electricity but also of xe2x80x9chotxe2x80x9d catalyst particles, i.e. fines. When ethylene is used as a comonomer, the xe2x80x9chotxe2x80x9d catalyst particles produce sticky, high ethylene containing polymer particles, xe2x80x9crubbery finesxe2x80x9d, in the rubber phase reactor, whereby fouling problems increase when the proportion of ethylene comonomers in the copolymer composition increases. In the other hand, the possibility to vary the ethylene comonomer content in the polymer allows a wide range of modifications. Therefore the fouling problem is not totally avoided in propylene polymerizing processes by using only an antistatic agent.
It is an object of the present invention to provide a process for polymerization of xcex1-olefins, especially propylene, in at least two polymerization stages, where the fouling is reduced, the process comprising a first stage for forming a polymer matrix (such as a homo/homo, homo/random or random/random (co)polymer matrix) in bulk and optionally in gas phase reactor(s) and additional stages for copolymerizing ethylene and xcex1-olefins in the presence of the polymer matrix from the first stage in at least one additional gas phase reactor, i.e. rubber phase reactor(s).
Another object of the invention is to provide a process for polymerization of xcex1-olefins, especially propylene using the above multistage process, which is easy to run continuously without uneconomical and troublesome cleaning breaks.
Still a further object of the invention is to provide a process for polymerization of xcex1-olefins, especially propylene, for producing xcex1-olefin polymers having uniform quality and desired properties, such as desired stiffness and impact strength.
Further, it is an object of the present invention to provide a process for producing xcex1-olefin polymers, which do not contain undesired residues of antifouling agents, and which can be used also e.g. in medical and food packaging applications.
It has now been found that in order to reduce fouling in xcex1-olefin, especially propylene multistage polymerizing processes, both the static electricity and catalyst activity in fines have to be reduced. Catalyst activity in fines has to be limited to such extent that the polymerization will not be discontinued, but the fouling effect of the tines is avoided. Stadis-type antistatic agents, which were discussed earlier in this specification, do not deactivate the active catalyst particles when used in allowable amounts and therefore they do not act as catalyst killers in the process. On the other hand it is known that ethanol is an effective catalyst killer. Ethanol would act also as an antistatic agent, but being a very effective catalyst killer or catalyst poison, its use has to be restricted to so small amounts in a polymerization process that the desired antistatic effect is not obtained. In connection with the present invention it has been found that by using a combination of suitable antistatic agent and a catalyst killer as an antifouling composition the desired aims can be achieved.
Further, it has been found that if the antifouling composition comprising antistatic agent and catalyst killer is introduced into any of the first stage reactors, the desired properties, especially the desired stiffness and impact strength of the polymer product are not achieved. Therefore, according to present invention the antifouling composition of the invention is introduced into the gas phase reactor(s) of the second stage, i.e. into the rubber phase reactor(s), in order to achieve the above desired properties of the xcex1-olefin polymer composition, preferably polypropylene composition. However, small amounts of the sole antistatic agent, but not the antifouling composition, can be fed, and is often needed to feed, to the first stage reactors.
More specifically, the present invention is characterized by what is stated in the characterizing part of claim 1.
An important benefit of the present invention is, that the polymerization process can be carried out more economically, because the run periods are longer due to the reduced need of the cleaning breaks. This results also in more uniform quality of the product.
Next, the invention will be examined more closely with the aid of the following detailed description.
According to the present invention xcex1-olefins are polymerized and the fouling is reduced in at least two stage process, wherein the first stage comprises the forming of the xcex1-olefin polymer matrix in at least one bulk reactor and optionally in at least one gas phase reactor and the second stage comprises copolymerization of ethylene and xcex1-olefins in the presence of said polymer matrix of the first stage in at least one gas phase reactor acting as a rubber phase reactor, and that an antifouling composition is fed into said rubber phase reactor.
As stated above it is an object of the present invention to prevent the fouling in multistage polymerization process of xcex1-olefins, especially in the (co)polymerization process of propylene with ethylene and if desired with other xcex1-olefins without impairing the properties, especially stiffness and impact strength properties of the polymer product.
According to the process of the present invention the polymerization is carried out in successive bulk and gas phase reactors in the presence of a high activity polymerization catalyst system. As a catalyst system is used a conventional catalyst system comprising a transition metal catalyst and as a cocatalyst an organometal compound, preferably an organoaluminium compound with the formula (I)
R3mxe2x88x92nAlmXnxe2x80x83xe2x80x83(I)
wherein R is a C1-C12 alkyl, X is halogen, m is 1 or 2 and n is an integer such as 0xe2x89xa6n  less than 3mxe2x88x921. Preferably, the first organoaluminium compound of the formula (I) is a tri-C1-C12 alkyl aluminium, most preferably triethyl aluminium TEA.
As stereoregulating external electron donor(s) is (are) preferably used hydrocarboxy silane compounds or hydrocarboxy alkane compounds. The more preferred external donors are di-C4-C12-hydrocarbyl-di-C1-C3-alkoxy silane or C4-C12-hydrocarbyl-C1-C3-alkoxy silane, and most preferred are dicyclopentyl dimethoxy silane or cyclohexyl methyl dimethoxy silane.
Hydrogen can be added both to the bulk and gas phase reactors for controlling the molar mass of the polymer per se.
The process of the invention is carried out in at least two stages as stated above. The first stage comprises at least one bulk reactor, preferably a loop reactor, and optionally at least one gas phase reactor, preferably one gas phase reactor. The second stage comprises at least one gas phase reactor, which is usually and also in this application called as a rubber phase reactor. It is possible that the first stage comprises only a bulk reactor, where the polymer matrix is formed, and the first gas phase reactor acts as a rubber phase reactor (the second stage). However, it is more usual, that the first stage comprises one bulk and one gas phase reactor. The polymer matrix is formed from xcex1-olefin homopolymers or copolymers with ethylene or other xcex1-olefins, as is defined earlier in this application. In the rubber phase reactor ethylene is copolymerized with the polymer matrix. By using the multistage polymerization processes above, the molecular weight distribution (MWD) of the homo and random copolymers can be broadened to obtain optimised processability. Further, products with very low to very high MFR (melt flow rate) values can be produced. Typically the polymer compositions, prepared with the present process, have wide range of the xylene solubles (XS), typically XS is more than 10, preferably 10 to 25.
According to one embodiment of the invention the polymerization conditions for the bulk reactor of the first stage are as follows:
the temperature is within the range of 40xc2x0 C. to 120xc2x0 C., preferably between 60xc2x0 C. and 100xc2x0 C.,
the pressure is within the range of 20 bar to 80 bar, preferably between 30 bar to 60 bar,
hydrogen can be added for controlling the molar mass in a manner known per se.
The reaction mixture from the bulk reactor is transferred to the gas phase reactor of the first stage polymerization. The polymerization conditions in the gas phase reactor are as follows:
the temperature is within the range of 50xc2x0 C. to 130xc2x0 C., preferably between 70xc2x0 C. and 100xc2x0 C.
the pressure is within the range of 5 bar to 40 bar, preferably between 25 bar to 35 bar,
hydrogen can be added for controlling the molar mass in a manner known per se.
The polymerization conditions in the rubber gas phase reactor (the second stage) are same as in the gas phase reactor of the first stage.
The melt flow rates, MFR, corresponding to the molecular weights of the polymer, can vary in wide ranges depending e.g. of the comonomer contents in the polymer. MFR of the polymer are measured according to ISO 1133 standard method. For polypropylene compositions MFR is measured according to said standard at 230xc2x0 C., with 2.16 kg load (MFR2). The polymer composition prepared according to the present process can have the MFR2 in the range of 0.03 to 2000 g/min, preferably 0.03 to 1000 g/10 min, most preferably 0.2 to 400 g/10 min.
According to the invention the antifouling composition used in the present process is a combination of an antistatic agent component and a catalyst killer component. This antifouling composition is fed into the rubber phase reactor(s) of the second stage in order to effectively prevent the fouling in the rubber phase reactor(s). In the rubber phase reactors the catalyst killer component of the antifouling composition of the invention reduces the catalyst activity in the fines, as is discussed above. Further, by feeding the antifouling composition to said second stage reactors the properties, e.g. desired stiffness and impact strength of the polymer composition, which were achieved during the first stage polymerization, will not be impaired, i.e. the desired level of these properties will be achieved. It has been noted, that if the antifouling composition were fed into the first stage reactors, the stiffness and impact strength properties tend to impair and the desired properties will not be achieved. However, small amounts of the sole antistatic agent component of the antifouling composition can be fed to the first stage reactors without effecting injuriously to the stiffness properties.
As the antistatic agent component in the antifouling composition of the invention is used a composition containing aromatic solvents (40-70 wt-%), light petroleum (10-30 wt-%), C1-6 alkyl alcohols (1-10 wt-%), sulfonic acids (5-30 wt-%), polymeric polyamines (5-20 wt-%) and polysulfone copolymers (5-30 wt-%). The preferred aromatic solvents are toluene and xylene, the preferred C1-6 alcohols are isopropanole and ethanol, sulfonic acids are preferably benzene sulfonic acids or naphtyl sulfonic acids, such as dodecylbenzenesulfonic acid or dinonylnaphtylsulfonic acids, and polysulfone copolymers can be e.g. 1-decene-polysulfone. A very useful antistatic agent component of the above type is an agent under the trade name Stadis 450, available by DuPont. As the catalyst killer component is used C1-6 alcohols, preferably C1-6 aliphatic alcohols, more preferably methanol, ethanol, propanol, or mixtures thereof. The most preferred alcohol is ethanol, which is non-toxic and easy and safe to use.
The ratio of the antistatic agent and catalyst killer in the antifouling composition used in the process of the invention is 1:4 to 4:1, preferably 1:2 to 2:1, by weight. The total amount of the antifouling composition to be incorporated to the rubber phase reactor in order to prevent the fouling but not effecting injuriously to the polymerization process itself is between 10-1000 wt-ppm, preferably 20-500 wt-ppm, more preferably 30-150 wt-ppm, calculated on the basis of the product obtained. The antifouling composition can be fed into the rubber reactor(s) continuously from an antifouling composition container by means of a pump or pumps.
By using the process of the invention the fouling the can be reduced considerably. The combination of the antistatic agent and suitable alcohol as the catalyst killer improves the operability of the process compared to that if only antistatic agent were used. By using ethanol in the amounts as stated above as the catalyst killer component the activity of the catalyst in fines can be kept under control. Further, changes in the ethanol feed give the response on catalyst activity in fines very fast allowing controlling the activity in the rubber phase reactor very easily. If, for instance, the catalyst activity is low in the loop reactor, like when running lower ethylene content or products with lower MFR, then higher ethanol feed to rubber phase reactor is required to keep reaction in desired level still in the rubber phase.